File:Lodge spark transmitter1894.png

English: An early spark radio transmitter built by British physicist Oliver Lodge for a lecture-demonstration before the British Royal Society 1 June 1894 on the occasion of Heinrich Hertz's death, which was later published as a book. It consists of a 5 in. metal ball resonator flanked on each side by spark gaps made by small metal balls all inside a short copper waveguide. When high voltage pulses from an induction coil are applied to the side balls, the voltage jumps the spark gaps, passing from one side ball to the other through the large center ball. The sparks excite brief standing wave oscillations in the center ball, which emits the energy as a pulse of microwaves which radiate out the aperture to the right. Lodge used the oscillator in the lecture to duplicate many of the experiments of Hertz demonstrating reflection, refraction, diffraction, and polarization of radio waves.